Les_Norton He said it was not locked 'by them' but it can be unlocked if I apply to Google to unlock it by providing IMEI numbers.
You were given false information (most likely by an inexperienced sales representative). It is not related to Google in that sense. It would entirely depended on the carrier.
This explained in the Prerequisites of our installation guide:
You need one of the officially supported devices. To make sure that the device can be unlocked to install GrapheneOS, avoid carrier variants of the devices. Carrier variants of Pixels use the same stock OS and firmware with a non-zero carrier id flashed onto the persist partition in the factory. The carrier id activates carrier-specific configuration in the stock OS including disabling carrier and bootloader unlocking. The carrier may be able to remotely disable this, but their support staff may not be aware and they probably won't do it. Get a carrier agnostic device to avoid the risk and potential hassle. If you CAN figure out a way to unlock a carrier device, it isn't a problem as GrapheneOS can just ignore the carrier id and the hardware is the same.